It has been known to simultaneously reproduce a plurality of content items each comprising video data and audio data. Such content items are reproduced in various environments, such as a general-purpose environment where a personal computer is used. In cases where a plurality of content items are to be reproduced simultaneously or synchronously, a plurality of videos to be simultaneously reproduced would be undesirably displaced from each other, a plurality of audio signals to be reproduced synchronously would be undesirably displaced from each other, or videos and audio signals would be undesirably displaced from each other, unless reproduction (playback) timing between the content items is adjusted or corrected appropriately. There have heretofore been known various techniques for synchronizing respective reproduction timing of content items.
Japanese Patent Application Laid-open Publication No. HEI-9-205618 (hereinafter referred to as “Patent Literature 1”) corresponding to U.S. Pat. Nos. 5,808,722 and 6,148,135 discloses, as an example of processing for synchronously reproducing a video and an audio signal, synchronizing a reproduction-controlling system clock and the video and audio signal by correcting the system clock to conform to or match a synchronizing signal of a picture frame of the video. However, with the known construction where synchronization processing is executed on the basis of a video as disclosed in Patent Literature 1, such synchronization processing would occur frequently depending on the reproducing environment. Because video data has a great data quantity and a high compression ratio, an enormous number of arithmetic operations would be required if the synchronization processing is executed frequently. Because of a heavy load of the aforementioned synchronization processing, content reproduction cannot be resumed immediately following the synchronization, so that the video cannot sometimes be reproduced smoothly. Particularly, in cases where a plurality of content items are to be reproduced simultaneously, synchronism between the content items may be undesirably lost due to the heavy load of the aforementioned synchronization processing. For example, in a case where one music piece (more specifically, an ensemble performance of one music piece) is to be reproduced with a plurality of content items, and particularly if reproduction of a plurality of audio signals of the content items is displaced from each other, reproduction of the music piece would fail musically. Thus, where a plurality of content items are to be reproduced simultaneously in a musical application, it is particularly necessary to secure synchronism between a plurality of audio signals.
Further, Japanese Patent Application Laid-open Publication No. 2009-10548 (hereinafter referred to as “Patent Literature 2”) discloses, as an example of processing for synchronizing a plurality of recorded data (coded video/audio data), a technique which performs a pattern matching comparison between sound volume levels of the plurality of recorded data (coded video/audio data) to extract a time difference between the plurality of recorded data and then records the plurality of recorded data in a synchronized state by correcting time information possessed by the individual recorded data in accordance with the extracted time difference. However, the technique disclosed in Patent Literature 2, which requires complicated operations for extracting sound volume levels of a given time period and then performing a pattern matching comparison between the extracted sound volume levels, is not suited for synchronization processing performed in real time during content data reproduction.
Furthermore, there have also been known multi-video reproduction apparatus which display a plurality of moving pictures (also referred to as video moving pictures) on a single screen. For example, Japanese Patent Application Laid-open Publication No. 2012-244343 (hereinafter referred to as “Patent Literature 3”) discloses a technique which automatically determines a layout, on the screen, of a plurality of video moving pictures on the basis of respective photographing information of the moving pictures and displays the video moving pictures on the screen in the determined layout. This Patent Literature 3 also discloses, as a method for changing the moving pictures displayed on the screen, starting reproduction of another video moving picture upon completion of the reproduction of any one of the moving pictures (preceding moving pictures) displayed on the screen. With this method, however, it is not possible to change over to another video moving picture before the reproduction of any one the preceding moving pictures ends.
Further, Japanese Patent Application Laid-open Publication No. 2001-36867 (hereinafter referred to as “Patent Literature 4”) discloses, as a method for additionally reproducing another video moving picture during reproduction of a plurality of video moving pictures in a multi-video reproduction apparatus, a technique in accordance with which a reproduction position of the other moving picture is moved in advance on a background in accordance with a current reproduction position of a plurality of moving pictures so that, in response to a display instruction, the other video moving picture can be displayed on the screen from the reproduction position having been moved to on the background as above. However, with this technique, where the reproduction position of the video moving picture to be added is moved in advance on the background, a user cannot newly designate a video moving picture during the reproduction of the plurality of video moving pictures.
Also, there have been known digital audio workstation (so-called “DAW”) systems constructed to be capable of performing a series of music-production-related processing, such as recording, editing and mixing of audio data (waveform data) and MIDI (Musical Instrument Digital Interface) data, by use of a general-purpose personal computer. Typically, on a GUI (Graphical User Interface) screen employed in such a DAW system, pictures representative of recorded audio data (waveform data) and MIDI events are arranged in a time-series order on a time axis extending in a horizontal direction of the screen, for each of a plurality of recording tracks arranged in a vertical direction of the screen (see, for example, a non-patent literature “Cubase 7 Operation Manual”, Steinberg Media Technologies GmbH, published on Feb. 13, 2012, available on the Internet at <http://japan.steinberg.net/fileadmin/redaktion_japan/documents/Cubase/Cubase_7_Operation_Manual_jp.pdf>. The aforementioned DAW system is designed to be capable of creating a musical work in a precise manner and thus difficult for an unaccustomed user, unfamiliar with a music performance or musical work production of a musical work, to use.
Furthermore, International Patent Application No. WO2008/069037 (hereinafter referred to as “Patent Literature 5”) corresponding to Japanese Patent Application Laid-open Publication No. 2008-139560 discloses an example of a music content creation system which comprises a server apparatus and client terminals interconnected via a communication network, and in which the server apparatus acquires audio and videos from a plurality of client terminals and creates one music content item by synthesizing the acquired audio and videos. According to the music content creation system disclosed in Patent Literature 5, one music content can be produced through cooperation of a plurality of participants, i.e. by the plurality of participants sharing production of a plurality of sections (such as an intro section, melody A section, melody B section, . . . ending section of a musical instrument) of the one music content. This music content creation system can be used easily even by a user unfamiliar with a music performance. However, this music content creation system is merely constructed such that a plurality of participants bring content corresponding to their assignments (content parts) like in so-called “collection of messages” and these content parts are synthesized by the server apparatus; it does not allow the individual participants to create and edit their musical works on their own or independently of each other, neither does it provide interfaces for such purposes.
Furthermore, Japanese Patent Application Laid-open Publication No. 2012-145643 (hereinafter referred to as “Patent Literature 6”) discloses an audio data recording/using system which records, on multiple tracks, a music performance executed in a rehearsal studio using recording facilities installed in the rehearsal studio and then not only stores the recording into a studio server but also uploads the recording to a shared server on the Internet so that the recording can be reproduced freely via any of user terminals connected to the shared server. However, this audio data recording/using system is based on the premise that a plurality of human players of musical instruments actually gather at the rehearsal studio, and thus, it requires various labor, such as one to gather a plurality of human players and to adjust schedules of the human players for the gathering.